1. Field of Invention
The invention is related to hydrometers and, more particularly, to box-style (box-type), swing-arm hydrometers used for measuring the concentration of dissolved salts in natural or artificial seawater.
2. Related Art
Hydrometers are instruments used to measure the density or specific gravity (also commonly referred to as relative density) of a liquid, i.e., the density of the liquid as compared to that of water when both are at the same temperature. Substances with a specific gravity greater than one are denser than water (and so will sink in it), and those with a specific gravity of less than one are less dense than water (and so will float in it). Hydrometers are often used, for example, by marine or brackish water aquarium enthusiasts who use natural or artificial seawater in their aquariums, or by people conducting field analysis of natural seawater.
Many types of seawater density measuring devices exist but generally may fall under three categories: (1) conductivity meters, which measure dissolved salts electronically; (2) refractometers, which measure density or salinity indirectly by passing light through the sample, using the refraction index of a liquid which is proportional to the amount of dissolved substances in the liquid; and (3) hydrometers, which come in several forms, but typically have some type of float that gives an indication of the water density based on a fixed scale. Each of these measuring devices must be calibrated for a specific temperature range, and each has its own advantages and disadvantages. Conductivity meters, for example, can be expensive, delicate instruments that need to be well maintained. They can also be difficult to operate correctly without proper training and, therefore, are generally not practical for use by the average aquarium hobbyist. Refractometers can be more practical, but require good eyesight and the reading must be taken quickly because evaporation of the sample can give inaccurate readings in a short period of time. Both conductivity meters and refractometers have a negative attribute in that they can be damaged by the effects of saltwater causing corrosion of metal parts.
Hydrometers are generally the most practical devices for use by aquarium hobbyists. Known hydrometers are typically sold as a device that has been calibrated and checked by the manufacturer. Thus, the user relies on the precision of the manufacturing for the level of accuracy and cannot calibrate the device manually. Several different types of hydrometers are known. One type of hydrometer, for example, includes a calibrated glass or plastic tube ending in a weighted portion that makes the tube stand upright when placed in a liquid. The lower the density of the liquid, the deeper the tube sinks. This type of hydrometer may typically contain a paper scale positioned inside the stem so that the specific gravity can be read directly based on the point at which the surface of the liquid touches the stem of the hydrometer. Such glass or plastic float hydrometers may be sufficiently accurate for aquarium hobby use, but it can be difficult to determine the correct reading because of the meniscus at the stem and the necessarily tiny print on the scale. Glass or plastic float hydrometers must also be calibrated by the manufacturer within a narrow temperature range, otherwise the readings will be off. Accurate glass or plastic float hydrometers can also be expensive and those made of glass may also be easily broken.
Another common type of hydrometer known in the aquarium trade is the box-style, swing-arm hydrometer. Box-style, swing-arm hydrometers are popular because they are easy to read, easy to use, and are relatively inexpensive. These hydrometers may typically include three components: (1) a box or tube; (2) an indicator arm (pointer) disposed in the box or tube and which points to numbers on a fixed scale; and (3) a counter weight disposed within the pointer. Differences in the dimensions of the box and/or the pointer as well as slight variations in the weight of the counter weight can all impact the accuracy of the hydrometer. Also, deposits of minerals on the pointer can make the accuracy change over time. Furthermore, if air bubbles become attached to the pointer as the hydrometer is being filled, the reading may not be accurate. Thus, although box-style, swing-arm hydrometers are generally designed to give fairly accurate readings across a wide range of temperature (called “temperature compensation”), they can have numerous disadvantages.